I’m using NextClouPi v.0.67.2 (NextCloud 14.0.4) and there I’ve executed the rc-init command.
Now all the settings are gone, but the files of the past users are all still there /media/USBdrive/ncdata/.
Now I would like to reassociate new users to the old file folders to make them visible on the files app.

Looks like you previously used the externa storage module, to place the user folders onto te external USB drive. If nothing else works… you still can use a symlink from /media/USBdrive/user1/ to /var/www/nextcloud/data/.

There are other, more involved options as well, but I’d check the external storage app first.